Cosmic structure growth and dark energy

Dark energy has a dramatic effect on the dynamics of the Universe, causing the recently discovered acceleration of the expansion. The dynamics are also central to the behaviour of the growth of large-scale structure, offering the possibility that observations of structure formation provide a sensitive probe of the cosmology and dark energy characteristics. In particular, dark energy with a time-varying equation of state can have an influence on structure formation stretching back well into the matter-dominated epoch. We analyse this impact, first calculating the linear perturbation results, including those for weak gravitational lensing. These dynamical models possess definite observable differences from constant equation of state models. Then we present a large-scale numerical simulation of structure formation, including the largest volume to date involving a time-varying equation of state. We find the halo mass function is well described by the Jenkins et al. mass function formula. We also show how to interpret modifications of the Friedmann equation in terms of a time-variable equation of state. The results presented here provide steps toward realistic computation of the effect of dark energy in cosmological probes involving large-scale structure, such as cluster counts, the Sunyaev–Zel'dovich effect or weak gravitational lensing.     

Robust Scalings in Compressible Flux Pile-Up Reconnection

Flux pile-up magnetic reconnection is traditionally considered only for incompressible plasmas. The question addressed in this paper is whether the pile-up scalings with resistivity are robust when plasma compressibility is taken into account. A simple analytical argument makes it possible to understand why the transition from a highly compressible limit to the incompressible one is difficult to discern in typical simulations spanning a few decades in resistivity. From a practical standpoint, however, flux pile-up reconnection in a compressible plasma can lead to anomalous electric resistivity in the current sheet and flare-like energy release of magnetic energy in the solar corona.     

Pan-African eclogite facies metamorphism of ultramafic rocks in the Shackleton Range, Antarctica

In the Shackleton Range of East Antarctica, garnet-bearing ultramafic rocks occur as lenses in supracrustal high-grade gneisses. In the presence of olivine, garnet is an unmistakable indicator of eclogite facies metamorphic conditions. The eclogite facies assemblages are only present in ultramafic rocks, particularly in pyroxenites, whereas other lithologies – including metabasites – lack such assemblages. We conclude that under high-temperature conditions, pyroxenites preserve high-pressure assemblages better than isofacial metabasites, provided the pressure is high enough to stabilize garnet–olivine assemblages (i.e. ≥18–20 kbar). The Shackleton Range ultramafic rocks experienced a clockwise P–T path and peak conditions of 800–850 °C and 23–25 kbar. These conditions correspond to ∼70 km depth of burial and a metamorphic gradient of 11–12 °C km⁻¹ that is typical of a convergent plate-margin setting. The age of metamorphism is defined by two garnet–whole-rock Sm–Nd isochrons that give ages of 525 ± 5 and 520 ± 14 Ma corresponding to the time of the Pan-African orogeny. These results are evidence of a Pan-African suture zone within the northern Shackleton Range. This suture marks the site of a palaeo-subduction zone that likely continues to the Herbert Mountains, where ophiolitic rocks of Neoproterozoic age testify to an ocean basin that was closed during Pan-African collision. The garnet-bearing ultramafic rocks in the Shackleton Range are the first known example of eclogite facies metamorphism in Antarctica that is related to the collision of East and West Gondwana and the first example of Pan-African eclogite facies ultramafic rocks worldwide. Eclogites in the Lanterman Range of the Transantarctic Mountains formed during subduction of the palaeo-Pacific beneath the East Antarctic craton.     

Estimation of snow water equivalent using microwave radiometry over Arctic first‐year sea ice

The magnitude and spatial distribution of snow on sea ice are both integral components of the ocean–sea‐ice–atmosphere system. Although there exists a number of algorithms to estimate the snow water equivalent (SWE) on terrestrial surfaces, to date there is no precise method to estimate SWE on sea ice. Physical snow properties and in situ microwave radiometry at 19, 37 and 85 GHz, V and H polarization were collected for a 10‐day period over 20 first‐year sea ice sites. We present and compare the in situ physical, electrical and microwave emission properties of snow over smooth Arctic first‐year sea ice for 19 of the 20 sites sampled. Physical processes creating the observed vertical patterns in the physical and electrical properties are discussed. An algorithm is then developed from the relationship between the SWE and the brightness temperature measured at 37 GHz (55°) H polarization and the air temperature. The multiple regression between these variables is able to account for over 90% of the variability in the measured SWE. This algorithm is validated with a small in situ data set collected during the 1999 field experiment. We then compare our data against the NASA snow thickness algorithm, designed as part of the NASA Earth Enterprise Program. The results indicated a lack of agreement between the NASA algorithm and the algorithm developed here. This lack of agreement is attributed to differences in scale between the Special Sensor Microwave/Imager and surface radiometers and to differences in the Antarctic versus Arctic snow physical and electrical properties. Copyright © 2003 John Wiley & Sons, Ltd.     

Titan's atmospheric engine: an overview

Heating occurs in Titan's stratosphere from the absorption of incident solar radiation by methane and aerosols. About 10% of the incident sunlight reaches Titan's surface and causes heating there. Thermal radiation redistributes heat within the atmosphere and cools to space. The resulting vertical temperature profile is stable against convection and a state of radiative equilibrium is established. Equating theoretical and observed temperature profiles enables an empirical determination of the vertical distribution of thermal opacity. A uniformly mixed aerosol is responsible for most of the opacity in the stratosphere, whereas collision-induced absorption of gases is the main contributor in the troposphere. Occasional clouds are observed in the troposphere in spite of the large degrees of methane supersaturation found there. Photochemistry converts CH₄ and N₂ into more complex hydrocarbons and nitriles in the stratosphere and above. Thin ice clouds of trace organics are formed in the winter and early spring polar regions of the lower stratosphere. Precipitating ice particles serve as condensation sites for supersaturated methane vapor in the troposphere below, resulting in lowered methane degrees of supersaturation in the polar regions. Latitudinal variations of stratospheric temperature are seasonal, and lag instantaneous response to solar irradiation by about one season for two reasons: (1) an actual instantaneous thermal response to a latitudinal distribution of absorbing gases, themselves out of phase with the sun by about one season, and (2) a sluggish dynamical response of the stratosphere to the latitudinal transport of angular momentum, induced by radiative heating and cooling. Mean vertical abundances of stratospheric organics and aerosols are determined primarily by atmospheric chemistry and condensation, whereas latitudinal distributions are more influenced by meridional circulations. In addition to preferential scavenging by precipitating ice particles from above, the polar depletion of supersaturated methane results from periodic scavenging by short-lived tropospheric clouds, coupled with the steady poleward march of the continuously drying atmosphere due to meridional transport.     

A non-coherent surface backscattering model for radar observation of planetary bodies and its application to Cassini radar altimeter

Mission and hardware constraints make the Cassini radar altimeter working in the beam limited or pulse limited mode dependent on the radar operative mode (Low and High Resolution, respectively), but never allows work in a condition such that the pulsewidth limited circle is much smaller than the beamwidth limited circle. Unfortunately this latter condition is vital for the application of the so-called Brown model widely and successfully used in Earth (ocean) observation missions where the quoted condition is really met. In the paper a new model is discussed which is based on the same general hypotheses of the Brown model but is worked out by means of a different approach which makes it more general and independent of the specific operative conditions. An extension of the new model to take into account large mispointing angles is considered as well based on a series expansion of the Bessel function and on the analysis of the truncation error. Finally a comparison with the classical Brown model is discussed too.     

Results from several persistent train spectra intercompared

Some results following from two contemporary photographical programs (Slovakia and Japan) for persistent meteor train spectra are compared. It shows that even though both programs are not too different according to their lenses and films used, the spectral interval detected is very different. In this respect prism as a dispersion element is more favourable than the grating with blaze wavelength near 610 nm. However, some results previously assumed as typicai for presented persistent trains-e.g. the forbidden Herzberg O₂ emission, the NO₂ continuum and OH red emissions-seem to be substanciated especially due to higher and linear dispersion of those grating spectra.